As textile substrates age, their color tends to fade or yellow due to exposure to light, air, soil, and natural degradation of the fibers that comprise the substrates. Thus, the purpose of bluing agents is generally to maintain the whiteness perception of these textile substrates and counteract the fading and yellowing of the substrates. Typically, bluing agents may be found in laundry detergents, fabric softeners, or rinse aids and are therefore applied to textile substrates during the laundering process. However, it is important that bluing agents function to improve the whiteness perception of treated textile substrates without causing undesirable staining of the textile substrates. Cellulosic substrates, in particular, tend to exhibit a yellow hue after exposure to light, air, and/or soiling. This yellowness is often difficult to reverse by normal laundering procedures. As a result, there exists a need for improved bluing agents which are capable of eliminating the yellowness exhibited by ageing cellulosic substrates. By utilizing such improved bluing agents, the life of the textile substrates, such as clothing articles, table linens, etc., may be extended. Domestic washes contain a mixture of fabric types and in the application of bluing agents care must be taken so that dye build up does not occur on fabric types other than cotton. Unfortunately, current bluing agents that deposit efficiently on cotton can deposit too strongly on nylon, especially at higher temperatures, thus over hueing the treated situs over multiple treatment cycles. There is a thus containing need for bluing agents that have improved selectivity for deposition on cotton over other fabric types, such as nylon.
The present invention offers advantages over previous efforts in this area, as this invention takes advantage of compounds having a non-sulfonic acid substituent in either the terminal and/or central phenyl ring of the bis-azo structure. Sulfonic acid groups are known to promote the deposition and staining of acid dyes on cellulosic fabrics. These groups are also essential for the solubility and compatibility of the dyes in laundry formulations. While it is necessary that bluing agents deposit from wash water, it is undesirable that they stain the fabric by inadvertent contact or by building up over time, i.e. overhueing. Applicants recognized that modification or replacement of the sulfonic acid group at the phenyl terminal end of the bis-azo with a nonionic solubilizing group allows for good deposition of the bis-azo but attenuates its staining and overhueing tendencies and still allows the bluing agent to be compatible in laundry formulations. Applicants further recognized that the selection of the terminal coupler significantly impacts the selectivity ratio and enables the intentional construction of dyes highly selective for cotton over nylon. In short, Applicants recognized the source of the current hueing deficiencies and herein provide the solution to such problem. The hueing compounds disclosed herein also absorb light at a wavelength appropriate to neutralize the yellowness of cellulosic substrates. These compounds function ideally as bluing agents for cellulosic substrates and may be incorporated into laundry care compositions for use by consumers.